Electronic device manufacturers strive to produce a rich interface for users. Conventional devices use visual and auditory cues to provide feedback to a user. In some interface devices, kinesthetic feedback (such as active and resistive force feedback) and/or tactile feedback (such as vibration, texture, and heat) is also provided to the user, more generally known collectively as “haptic feedback” or “haptic effects”. Haptic feedback can provide cues that enhance and simplify the user interface. Specifically, vibration effects, or vibrotactile haptic effects, may be useful in providing cues to users of electronic devices to alert the user to specific events, or provide realistic feedback to create greater sensory immersion within a simulated or virtual environment.
In order to generate vibration effects, many devices utilize some type of actuator or haptic output device. Known haptic output devices used for this purpose include an electromagnetic actuator such as an Eccentric Rotating Mass (“ERM”) in which an eccentric mass is moved by a motor, a Linear Resonant Actuator (“LRA”) in which a mass attached to a spring is driven back and forth, or a “smart material” such as piezoelectric, electro-active polymers or shape memory alloys. Haptic output devices also broadly include non-mechanical or non-vibratory devices such as those that use electrostatic friction (“ESF”), ultrasonic surface friction (“USF”), or those that induce acoustic radiation pressure with an ultrasonic haptic transducer, or those that use a haptic substrate and a flexible or deformable surface, or those that provide projected haptic output such as a puff of air using an air jet, and so on.
Development of haptic feedback structures has led to smaller, more compact devices. As display screens having high definition have increased, so has the need for high definition (“HD”) haptic feedback. Piezoelectric based linear resonant actuators may be the next generation HD haptic actuators for touch screen surfaces and other applications. Due to its nature of being a resonant actuator, a piezoelectric based linear resonant actuator can create a haptic feedback that is a little “buzzy”, which is caused by the low damping ratio of the actuator so that after the actuation signal ends, the piezoelectric linear resonant actuator will continue to vibrate for some time. It is desirable to terminate this vibration residue as fast as possible. Although closed loop control strategies have been used to reduce the “buzzing”, closed loop control strategies require sensing and some computational ability in real time, which adds cost to the actuator.